Multi-dimensional fabrication by joining and/or material healing (repair) is limited, in part, due to inherent scale-based challenges in available tools. Most common defect repair or joining techniques like soldering and welding, are limited to bulk uses and cannot be adopted to smaller sizes especially at the microscale and smaller.
Ability to join, or fuse, materials is ubiquitous to manufacturing in many fields such as; electronics, chemical, energy, medical, aerospace, defense, among others, but has been facing challenges, in part, due to increased interest and advances in miniaturization, coupled with the need for greener processes[1,2]. Nanotechnology, for instance, has grown significantly in the recent past, and produces high performance materials with many desirable properties. There are, however, obstacles to fully actualizing the potential of nanomaterials because of limitations in fabricating complex structures and devices[3]. Recently, there has been efforts for interconnections of nanomaterials by welding (e.g. cold welding[4], fusion welding[5], plasmonic welding[6]), soldering (e.g. liquid-phase reflow soldering[7], resistance soldering[8]), brazing[9] and others processes[10] that heavily rely on in situ manipulation, directed assembly, and, self-assembly. The techniques, however, suffer from low efficiency, high costs, and often need specialized samples (e.g. contamination-free, flat surface, and, high purity depending on the technique), as such, they are far from adoption in large scale manufacturing. Similarly, for microsystems, joining is anticipated to be a significant hurdle in their adoption in large-scale manufacturing and fabrication.[1, 11]
In addition to miniaturization issues, the joining industry has been facing two other challenges, viz; (1) Despite the well-established lead-bearing solders that have been used extensively in the assembly of modern electronic devices, limitations of lead use due to environmental and health concerns has triggered research on alternative lead-free solders. Lead-free solders, however, often require higher processing temperatures than lead-containing solders (>450 K) which limits their use and increases cost. (2) Developing flexible electronics, polymer based substrates, electronic devices, and, temperature sensitive components (such as LEDs) require creating joints at low processing temperatures[12, 13]. Also, demand for less energy consuming or more energy efficient processes has been increasing. Therefore, practical energy efficient joining and manufacturing techniques, with low processing temperature, enabling fabrication of complex structure at the micro and smaller size scales is essential for future developments in device manufacturing. It has previously been suggested that metal nanoparticles can be used as a solder material since the melting temperature decreases with reduction in particle size.[13, 14]
Undercooling of metals (i.e. cooling of a liquid metal or alloy below its freezing point without it becoming solid, also known as “supercooling”) has been widely studied, primarily to inform metal processing and microstructure evolution during solidification.[15, 16] Due to the metastable nature of undercooled metals, their production in good yields is an experimental challenge. This challenge can be overcome through; i) elimination of heterogeneous nucleating sites, or other sites with high catalytic potency for solidification, and, ii) minimizing the container effects by employing the droplet dispersion or containerless techniques in synthesis of undercooled particles.[17, 18] Using these techniques, undercooling values as high as about 0.3-0.4 Tm have been reported.[16, 19] One of the highest undercooling achieved so far is 0.7 Tm for 3-15 nm gallium particles.[20] The literature on undercooling, however, is heavily skewed towards studies on understanding the solidification behavior and thermodynamics of metal systems.
There is limited discussion on practical applications except for heat transfer[21] and production of metastable solids.[17] One of the reasons for lack of practical use could be the challenges in preparing stable undercooled particles in high yields and at any size scale especially where large undercooling values are desired. In the container-less drop tube technique, for example, the particle is undercooled during free fall. Droplet emulsion techniques, on the other hand, allow for the production of more than one particle at once only if the carrier liquid can maintain a thin, inert surface coating inhibit crystallization, however, stability is still a major concern.
A so-called Shearing Liquids Into Complex ParticlEs process (known as the SLICE technique)[13] involves use of a rotating implement to shear a liquid metal, that is liquid at room temperature, into smaller pieces in an acid containing carrier fluid as illustrated schematically in FIG. 1. Under SLICE, the liquid metal is sheared to the desired size with concomitant surface oxidation to give a passivating layer on which an organic layer is assembled to give smooth surfaces, which is a key component in efficient particle assembly.
The SLICE process is described in PCT/US14/69802 filed Dec. 11, 2014, and a related technique has been reported in U.S. Pat. No. 4,042,374 issued Aug. 16, 1977